Low density foam, such as polystyrene foam, is commonly made by combining a physical blowing agent with a molten polymeric mixture under pressure and, after thorough mixing, extruding the combination through an appropriate die into a lower pressure atmosphere.
From about the 1950's to the present, physical blowing agents of choice have included halocarbons, hydrocarbons or combinations thereof. Examples of these include commercially available halocarbon compositions such as dichlorodifluoromethane, trichlorofluoromethane and mixtures thereof, and the C.sub.2 -C.sub.6 hydrocarbons. During the 1980's, the worldwide scientific community presented sufficient evidence linking chlorofluorocarbons (CFCs) with atmospheric ozone depletion and sought governments to regulate CFCs. As of a result of such regulations, hydrocarbons are generally the choice of physical blowing agents.
There are two foams that are commonly produced. The first foam is made from polystyrene and the second foam is made from low density polyethylenes (LDPEs). Pure polystyrene foam is too brittle for some applications such as protective packaging which require protection from multiple impacts.
LDPE foams are generally considered to be resilient and non-brittle, which are desirable properties. The LDPE foams, however, have disadvantages such as adding a stability control agent (also referred to as a permeation modifier) to the polymeric composition so as to produce a commercially acceptable foam (e.g., a foam that does not change its dimensions significantly over time).
The amount of total residual blowing agent in the LDPE foam immediately after its manufacture is typically in the range of from about 5 to about 10 weight percent of the polymeric composition. This amount is dependent upon factors such as is the desired density of the foam and the selected blowing agent. This amount of total residual blowing agent generally produces a potentially flammable condition if the foam is located in a confined area. Typically, the aging process for an LDPE foam containing a stability control agent takes from about 14 to about 30 days. The aging process is dependent upon a number of factors including, but not limited to, the density of the foam, the selected blowing agent and storage temperature of the foam.
Accordingly, a need exists for foams that overcome the above-noted shortcomings associated with existing foams.